During an earthquake or a blast from an explosion, a building is subjected to cyclic loading in the form of repeated tensile and compressive forces. Buckling restrained braces (BRBs), also known as unbonded braces, are finding acceptance as structural elements that add reinforcement and energy dissipation to steel frame buildings to protect the buildings against large deformations induced by earthquakes or blasts from explosions. The brace is designed to yield in tension or compression while resisting buckling.
A prior art BRB employs a steel core and a steel casing. The steel core has a yielding segment, typically provided by a narrowed or necked region. The casing prevents buckling of the core. Concrete or mortar fills the space between the core and the casing. The core cannot bond to the casing, so an unbonding layer, such as a TEFLON layer, may be applied over the core.
The buckling restrained brace absorbs seismic energy while mitigating inter-story drift. Performance-based design of earthquake resistant buildings requires technologies that can simultaneously minimize inter-story drift and floor accelerations. While inter-story drift is always taken into account by design engineers, protection against floor accelerations is often overlooked. Inter-story drift causes damage to a building's framing, facade and windows. Floor acceleration causes damage to ceilings, electrical systems, elevators, and building contents in general. Viscous and hysteretic dampers are technologies which provide energy dissipation with the ability to greatly reduce inter-story drift, but with minimal impact on reducing floor accelerations. BRBs, on the other hand, provide both energy dissipation and added stiffness with the ability to deform plastically, thereby reducing both inter-story drift and floor accelerations. The more powerful the earthquake, the greater the inter-story drift—and thus the greater the brace displacement—that needs to be accommodated. The extent to which floor accelerations may be mitigated depends on the brace's yield strength.
Advantages of BRBs over conventional braced frames include smaller beam and foundation design, control of member stiffness, greater energy dissipation, and reduced post-earthquake maintenance. The added cost of BRBs (such as additional development, materials, and transportation) may therefore be offset by savings in foundation and overall frame design. Current market trends seem to be moving away from damping and toward higher stiffness and very high purchased BRB capacities, from 200 kips at the low end to greater than 1000 kips.